I guess we'd better be sure we are all talking about the same thing. These are chamber headspace gages:
This Precision Mic and Hornady LNL caliper adapter are case headspace gages. The photo is of an improvised one showing the case to be 0.003" larger than the chamber headspace gage the caliper was zeroed on:
The chamber headspace gages are used to check a firearm's headspace. The case headspace gages are used to check whether or not you are setting a shoulder back resizing and if so, how much. It is an indirect measure of chamber headspace, but because cases spring back a little inside the chamber after firing, it can be off a couple of thousandths unless you neck size and fire it repeatedly until it is a little more snug in the chamber fit. That's not as easy to do, as it usually requires a more experienced sense of feel than the solid steel gage does.